Ionic vacuum pump



g- 1967 R, P. MOKEEVER IONIC VACUUM PUMP 2 Sheets-Sheet 1 Filed March 22, 1965 INVENTOR BY Z44 @Zs-Q Aug. 29, 1967 R. P. MCKEEVER IONIC VACUUM PUMP 2 Sheets-Sheet Filed March 22, 1965 INVENTOR.

Richard P McKeever Attorneys 3,338,507 IONIC VACUUM PUMP Richard P. McKeever, Palo Alto, Calif., assignor, by mesne assignments, to The Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New York Filed Mar. 22, 1965, Ser. No. 445,835 12 Claims. (Cl. 230-69) ABSTRACT OF THE DISCLOSURE Ion vacuum pump employs concentrically arranged electrodes with an electron emission source disposed and arranged to be maintained at a predetermined potential substantially corresponding to the potential of that one of the imaginary surfaces of revolution defined about the central electrode which represent a potential gradient of gradually diminishing value progressing radially outwardly of the central electrode. The electron emission source serves to restrict the circumfusion of electrons into the space to be pumped primarily to planes substantially normal to the axis of the electrodes.

This invention relates to an electronic getter ion pump, and in general it is an object of the present invention to provide an improved getter ion pump.

Certain getter ion pumps employ electron emission wherein a stream of electrons is injected into a space to be pumped for the purpose of ionizing gas. Once the gas is ionized it is impelled by an electric field into the getter film and buried there.

In getter ion pumps of the above kind where the stream of injected electrons will follow a cycloidal path between concentric cylindrical electrodes under an applied electric field existing therebtween, the electrons contact and ionize gas molecules in the space between the electrodes. The length of the cycloidal electron path is preferably as long as possible so as to increase the chances of collision between the electrons and gas molecules in the space being pumped. Means are provided herein for increasing the length of the path of electrons emitted into the space being pumped. Means are also provided herein for increasing the number of electrons in flight within the pump.

It is considered that a stream of electrons as, for example, might emanate from an electron gun, provides an unduly localized space charge which adversely foreshortens the path of the electrons being emitted. Where concentric cylindrical electrodes are employed at different potentials there will exist between the two electrodes imaginary concentric cylinders of equal, but progressively diminishing or increasing potential radially of the axis of the two electrodes depending on which is the anode and which the cathode. Thus, if the outer electrode is maintained at a potential less than the potential of the inner electrode, the potential of the imaginary concentric cylinders of equal potential will progressively decrease with radially outward displacement from the inner electrode.

If the electrons are injected into the space to be pumped in such a manner as to disturb the otherwise cylindrical imaginary surfaces of equal potential, a bump in the potential gradient between the two concentric electrodes will occur which will influence the electron trajectory and foreshorten the electron path thereby decreasing the chances of collision between the emitted electrons and gas in the pumping space.

It is therefore an object of the invention to inject electrons into a space which is to be pumped with as little disturbance of the potential gradient existing between the inner and outer electrodes as possible.

I have observed, as explained in detail later herein, that if the source of electrons being injected employs circum- United States Patent fusion of the electrons, (i.e., an omnidirectional electron emission from a source), which is limited to those planes disposed substantially normal to the axis of the inner electrode, a greatly extended electron path'will be achieved for the emitted electrons.

It is a further object of the invention to provide an improved electron emitter for injecting electrons into a getter ion pump.

These and other objects of the invention will become more clearly apparent from the following detailed description of a preferred embodiment, when taken in conjunction with the drawings in which:

FIGURE 1 is a diagrammatic illustration of a getter ion pump according to the invention;

FIGURE 2 is an enlarged schematic view of an electron source employed in a getter ion pump according to the invention;

FIGURE 3 is an elevation center line section view of a getter ion pump according to the invention;

FIGURE 4 is a bottom view in transverse section taken on the line 4-4 of FIGURE 3; and

FIGURE 5 shows an enlarged schematic view of another embodiment of an electron source.

In general, there is provided an ionic vacuum pump having an outer cylindrical electrode adapted to receive a surface application of gettering material which can be applied by sublimation from a source of such material employed within the pump. An inner cylindrical electrode is provided concentrically disposed within the outer electrode and the inner electrode is adapted to be maintained at a relatively high potential which is positive with respect to the outer electrode, whereby the inner electrode becomes the anode and the outer, the cathode. This manner of biasing the inner electrode provides imaginary concentric cylinders of equal potential, the potential of each becoming progressively less with radially outward displacement from the inner electrode. A source of gettering material to be sublimated by electron bombardment emanates from-around the inner electrode. A source of electrons is provided serving to ionize gas within the space defined by the outer electrode and also serving to bombard the source of getter material, such as the surface material of the anode.

Electron emission means is provided serving to circumfuse electrons primarily in planes substantially normal to the axis of the inner electrode (anode).

The electron emitting source comprises in general an elongated filament adapted to be maintained at a predetermined potential preferably a few volts positive with respect to the outer cylinder. The axis of the filament is spaced substantially parallel to the axis of the anode and an electrode encircles the filament. The last named electrode can be referred to as a field control electrode and serves to accelerate electrons emitted by the filament. Therefore, it is adapted to be maintained at a positive potential with respect to the filament so as to develop the desired electron energy for causing electron emission from the filament. The field control electrode is formed with respect to the filament so as to pass the emitted electrons in planes substantially normal to the axis of the inner electrode. Further, the axis of the field con-- trol electrode is disposed substantially on the imaginary surface of that one of the imaginary cylinders having a potential substantially corresponding to the potential of the field control electrode to thereby minimize disturbance of the potential gradient of the field provided radially between the inner and outer electrodes.

As more particularly shown schematically in FIGURE 1 the getter ion pump 10 comprises a cylindrical enclosure 12 forming an outer electrode and a cylindrical rod-like electrode 14 forming an inner electrode. The inner electrode 14 is adapted to be maintained at a relatively high voltage positive with respect to the outer electrode and therefore is herein referred to as the anode 14. This is schematically illustrated by the presence of lead 16 and the three plus signs associated therewith. The outer electrode is shown schematically with a lead 18 and a zero indicating ground potential.

Means communicating with the space within electrode 12 is provided whereby the pump can be associated with a space to be evacuated. Thus the lower end of cylinder 12 can provide communication with a space to be pumped as is shown in the construction of FIGURE 3.

A source of getter material to be sublimated by electron bombardment and deposited upon the inner wall of electrode 12 includes anode 14 of suitable getter material, such as titanium. Anode 14 is suitably supported coaxially of cylinder 12 preferably as described in further detail below.

An emitter assembly 29 provides a source of electrons characterized by omnidirectional electron emission limited primarily to planes substantially normal to the axis of anode 14. Emitted electrons are also accelerated by a field control electrode 38 described below.

A cylindrical insulating body member 30 supports an elongated cathode filament 32 axially thereof. Filament 32 is formed as a helical strand of wire which terminates in a pair of leads 34, 36 extending through and out of the upper end of body member 30.

A strand of wire forming an electrode 38 is wrapped tightly around the lower end of body member 30 so as to be supported coaxially of the helical filament 32. Electrode 38 is formed as a helix of opposite hand to the helix of filament 32. The pitch of the two helices serves to cause the strands of wire which respectively form filament 32 and electrode 38 to be substantially continuously mutually disposed at roughly right angles to one another throughout their coaxial extent. It is considered that this arrangement for an electron source provides maximum transmissivity of electrons through the electron accelerating electrode, as well as to cause the electrons to be emitted initially primarily in planes substantially normal to the axis of filament 32. An insulating washer 35 passes a lead wire upwardly to be taken out of the sealed vacuum enclosure.

As noted above, the electron emission is primarily limited to planes which are substantially normal to the axis of anode 14. It has been observed that the magnitude of the total momentum is fixed. Therefore, momentum in the axial direction is gained only at the expense of momentum in directions which lie in planes perpendicular to the axis of the central cylinder (anode 14). Thus, angular momentum is lost when axial momentum is gained, and conversely, momentum in the axial direc tion is gained only at the expense of angular momentum.

An electron having an angular momentum below a certain value will strike the central cylinder (anode 14) before it reaches the perigee of its orbital path, and hence, will prematurely terminate its orbit. Thus, for example, based on the above observation, a spherical source of electrons will emit a high percentage of electrons having a large amount of axial momentum (with a commensurately small amount of angular momentum) so that a large portion of the electrons will fail to orbit, whereas by limiting emission to planes substantially normal to anode 14, axial momentum is minimized and angular momentum maximized.

Circumfusion of the electrons emitted, limited to those planes generally perpendicular to anode 14, provides a relatively uniform space distribution of electrons so as to avoid adverse localized space charge forces that would otherwise tend to disturb and foreshorten the orbit of electrons being emitted as well as for those already in orbit.

By disposing assembly 29 with filament 32 on an axis substantially parallel to the axis of anode 14 the electrons emitted by filament 32 are circumfused from filament 32 equally in all directions but are substantially limited initially to planes generally perpendicular to the axis of anode 14. As will be explained in detail further below, certain reflecting fields in the pump serve to cause emitted electrons to migrate along the length of the anode, notwithstanding that the electrons are initially introduced in planes substantially perpendicular to the pump axis.

By slightly tipping the axis of assembly 29 on the order of a degree or two with respect to the anode axis, further downward spiraling of electrons can be developed. Therefore, as used herein, the term substantially perpendicular contemplates certain modest deviations on the order of the above.

Accordingly, the electrons are circumfused from the filament equally in all directions but substantially limited to planes substantially perpendicular to the axis of anode 14 so as to allow a relatively uniform space distribution of electrons and thereby avoid localized space charge forces which would otherwise tend to disturb the orbits of electrons.

Further, it is considered that a larger total number of electrons orbiting at any given time in the space within cylinder 12 can be obtained. The larger central rod provides greater capacitance whereby a larger charge will exist on the electrodes for the same voltage. Thus, a greater number of electrons which constitute a larger circulating charge can be put in orbit without adversely neutralizing the field created between the electrodes 12, 14. On the other hand it is further considered that the relatively smaller localized space charge forces will dis turb the electron orbits to a much lesser degree and thus permit a longer average path length from the electron source to the central (anode) cylinder, 14.

Another embodiment of an emitter assembly is shown in FIGURE 5. A cylindrical insulating body member 50 supports an elongated linear cathode filament 52 axially thereof. Filament 52 is formed as a single strand of Wire folded upon itself to form a pair of leads 53, 54 extending through and out of the upper end of body member 50.

A field control electrode 56 is carried from the lower end of body member 50 and constructed whereby elements thereof are disposed substantially continuously at roughly right angles to filament 52. Thus, the field control electrode includes a pair of conductive frame wires 57, 58 depending downwardly from the bottom outside margin of body member 50 and continued to include a lead 59. Frame members 57, 58 serve to support a plurality of annular conductive rings 61 spaced longitudinally along the axis of the assembly.

When installed to cooperate with anode 14, emitter assembly 29 is disposed radially spaced from anode 14 whereby the axis of control grid 38 lies substantially on the imaginary surface of that one of the imaginary cylinders of equal potential which has a potential substantially corresponding to the potential of electrode 38. This disposition serves to minimize disturbance of the potent1al gradient of the electric field radially between the inner and outer electrodes 14, 12, respectively. The electrode 38 is thereby adapted to be maintained at a bias potential corresponding to the potential of the imaginary cylinder indicated by reference numeral 40.

As shown schematically in FIGURE 1 emitter assembly 29 is disposed to lead into the upper end of pump 10. Body member 30 extends downwardly from the upper end of cylinder 12 in a space to be pumped so as to dispose the common axis 46 of filament 32 and electrode 38 upon the imaginary surface of cylinder 40.

By way of background explanation, in order to obtain the optimum performance of those ion pumps employing an exposed filament not having a field control electrode element as herein, the filament must be placed close enough to the anode, such as the central electrode 14, so as to cause it to emit electrons in sufficient quantity to cause adequate heating of the central titanium cylinder or rod (e.g. 14). Also electrons leaving the filament must be given sufiicient angular momentum such that most of them miss the central rod on their first orbit. The angular momentum is proportional to the radial position and the tangential velocity. The closer the exposed filament source is to the central cylinder, the larger the potential hump it introduces and therefore the larger the tangential velocity acquired. However, the radial displacement is correspondingly smaller. Accordingly, in such pumps the positioning of the exposed filament represents a compromise between radial position, bias, emission, angular momentum, and severity of the potential hump.

From the foregoing discussion it can be seen that the disposition of the filament in those ion pumps employing an exposed filament places the filament in a higher electric field, i.e. places the filament relatively close to the axially placed anode of the pump.

In such devices, due to the bias which must necessarily be placed on the filament, the imaginary equipotential surfaces in the region of the filament necessarily are distorted from their otherwise cylindrical symmetry. The equipotential surfaces are relatively severely indented in the region of the filament. Consequently electrons al ready in orbit experience forces which change their angular momentum about the axis of the pump. The change in angular momentum will lead to a foreshortened trajectory for a significant number of the electrons entering the region of the filament. It is intended that these indentations, be minimized, as by the apparatus disclosed herein.

It has been further observed that the indentations as described above with respect to devices serve to cause a large proportion of emitted electrons to move initially in a tangential direction. This condition serves to develop localized space charge forces which tend to further foreshorten the trajectories of orbiting electrons. Where electron sources emit beams or sheets of electrons, similar limitations have been observed.

If the exposed filament of such prior devices is spaced too far from the axis of the anode the emission becomes diflicult to obtain. The tangential component of the emitted electrons cannot be made large enough to give the emitted electrons sufficient angular momentum to cause them to miss the central anode on their first orbit. characteristically, therefore, in the past such filaments have been located at a radial position roughly 0.3-0.4 of the distance from the center of the pump.

As disclosed herein, however, the emission, angular distribution and initial energy of the electrons emitted from the electron source can be made independent of the radial position of the source. It has been observed that if sufficient emission and angular momentum can be obtained at any given radial displacement of the electron source from the axis of the anode, the optimum relationship of the radial displacement of the source from the anode can be determined to be something on the order of y ths of the displacement of the inner surface of the outer cylinder from the axis of the anode. Ideally, to sweep out the entire volume between anode 14 and cathode 12, the filament is biased on the order of a few volts positive with respect to the outer cylinder (cathode) sufiiciently to prevent most of the emitted electrons from striking the outer cylinder.

More particularly, using the notations r as the radial displacement of the electron source from the central axis of the concentric electrodes 12, 14; r as the radial displacement of the surface of the central anode 14; and r as the radial displacement of the inner surface of the outer cylinder 12, it has been calculated that if sufficient emis sion and angular momentum are to be obtained at any given r then the optimum r /r is close to 0.6. Location of assembly 29 to dispose axis 46 at a disposition where r /r approximates 0.6 improves performance of the pump, in the sense of improved pumping of gas which is pumped as a result of ionization and excitation of gas molecules 6 as distinguished from gettering of neutral and unexcited gas molecules.

The preferred disposition of the electron source 29 at substantially ii ths of the radial distance from the center of the axis of anode 14 to the inner surface of electrode 12 has been observed to permit optimum ion pumping with the further advantageous use of a larger practicable diameter anode where the anode itself is of the type which serves to provide the source of gettering material. Thus, a larger diameter anode, where supplying getter material, will provide a larger source of getter material. At the same time, the enlarged anode advantageously increases the capacitance between the inner and outer electrodes.

The advantage of an increased capacitance is due to the attendant ability to thereby increase the number (concentration) of circulating electrons without neutralizing the effect of the charges on the anode and the outer cylinder which form the electric field between the two cylinders. In this way also, then, the chances of collision between the electrons and gas molecules are enhanced in proportion to the increase in electron concentration. In short, all other things remaining constant, the chances of collision between the electrons and gas molecules are enhanced in proportion to this increase in capacitance.

A preferred embodiment of an ionic vacuum pump is shown in detail in FIGURE 3 and comprises an outer cylindrical electrode in the form of the body shell 72. The inner surface of body shell 72 is adapted to receive an application of gettering material deposited thereupon. Both the upper and lower end of body shell 72 are formed with encircling flanges 73, 74 respectively. Each of flanges 73, 74 is provided with an annular sealing surface 76, 77 respectively whereby a gasket seal 78 can be disposed to provide a high vacuum seal with a coacting member having a sealing surface comparable to the sealing surfaces 76, 77.

The upper end of body shell 72 is provided with a high vacuum closure member 79 formed with a sealing surface 81 disposed to bear upon the high vacuum sealing gasket 78. Closure member 79 forms the base for supporting a pair of electron emission assemblies 82, 83 comparable, for example, to those shown in FIGURES 2 or 5.

A pair of lead-through devices 84, 85 are carried to pass through closure member 79 for applying a predetermined filament potential to the filament of each emitter assembly 82, 83. The filaments of each emitter assembly 82, 83 are electrically connected via lead 86. The leadthr-ough devices 84, 85 are connected, as by suitable clips 88 to their inner ends. Emitter assemblies 82, 83 are carried by a support plate 91 formed with a pair of downwardly depending sleeves 92, 93. Assemblies 82, 83 are respectively disposed coaxially within sleeves 92, 93 respectively and are carried by plate 91 in any suitable fashion. Each emitter assembly 82, 83 further includes a control electrode 94, 96 respectively. Electrodes 94, 96 are interconnected by a lead 97 (FIGURE 4) and brought out via a pair of lead-through devices 98, 99. Lead-through devices 98, 99 are comparable to devices 84, 85 for making electrical entry into the sealed interior of the space enclosed by body shell 72.

Closure member 79 further serves to support an anode 101 of gettering material coaxially of body shell 72. Anode 101'is preferably arranged as now to be described.

A high voltage lead-through element 102 is shown partially broken away in FIGURE 3 whereby it can be seen that the upper end of anode 101 is received in a bore 103 formed axially along a downwardly depending sleeve portion 104 formed on the lower end of a conductive rod 106. Rod 106 forms a part of the lead-through device 102. The upper end of anode 101 is releasably engaged within bore 103 by a screw 107 radially screwed against the upper end of the anode. Access to screw 107 is provided radially through an opening 108 formed through the side of a downwardly depending hollow support stud 109 carried from the underside of closure member 79. A metal disc 112, or flange, is secured to the lower end of sleeve portion 104 so as to block getter material from entering spaces above and contaminating lead-through device 102. Stud 109 is further formed with a downwardly extending sleeve 113 adapted to extend coaxially in spaced relation along anode 101.

The lower end of body shell 72 is adapted to open into an area to be pumped. Accordingly, by conventional means, not shown, high vacuum closure can be formed in cooperation with the closure surface 77.

In operation electrode 14 is biased with a relatively high voltage potential, positive with respect to electrode 12. Electrode 38 is biased positive with respect to the filament 32 to provide the emitting electrons with a sufficient emission energy to cause them to be circumfused into the space between anode 14 and electrode 12. The circumfusion is primarily limited to planes substantially normal to the axis of anode 14. As the electrons follow their respective trajectories they will collide with, and ionize gas molecules within the space enclosed by the outer electrode 12. Resultant ions will be impelled by the existing electric field to become embedded into the surface of electrode 12. Most of these ions therfore will be permanently entrapped. Most of the electrons will also bombard the titanium source of getter material 14 thereby providing a fresh film of getter material upon the inner wall of electrode 12 to further trap gas.

It will be apparent that ionic pumping of the space defined between the substantially concentric cylindrical electrodes follows the steps of biasing the first electrode positive with respect to the second electrode to form imaginary cylinders of equal potential between the two electrodes. The potential of each imaginary cylinder becomes less progressively with radial outward displacement thereof from the inner electrode. The next Step includes circumfusing electrons within the space between the first and second electrodes so as to bombard the first electrode and sublimate getter material from the inner electrode to the inner surface of the outer electrode. Steps are taken to restrict the circumfusion of electrons primarily to planes substantially normal to the axis of the two concentric electrodes.

The filament of the emitter assembly 29 is ideally biased sufficiently positive with respect to the outer cylinder to prevent most of the electrons from reaching the outer cylinder. This allows electron paths to sweep out substantially the full volume. This also allow a smaller concentration of electrons for the same total number of electrons. Space charge forces are thereby decreased and thus, the average length of an electron path is increased.

As shown in FIGURE 3, although the electrons are initially introduced in planes substantially perpendicular to the axis of anode 101, they will proceed downwardly under the influence of reflecting fields 115, 116. Fields 115, 116 exist between sleeves 92, 93 and associated grids 94, 96, respectively. Later, as the electrons approach another field 117 existing between sleeve 113 (at ground) and anode 101, they will be further influenced to travel downwardly within cathode cylinder 72. Further, for those electrons being emitted at the lower end of emitter assemblies 82, 83 Which may be substantially beyond the influence of fields 115, 116, the electrons, to the extent that they are not moving precisely perpendicular to the axis of anode 101, will spiral downwardly. Any electrons introduced in planes tilted upwardly will immediately encounter fields 115 and 116 so that they will also be spiraled downwardly thereafter.

At the bottom end of anode 101 electrons will be reflected upwardly by an electric field 118 existing between anode 101 and cathode cylinder 72. Terminating the anode 101 short of the end of cathode 72 satisfactorily provides field 118.

From the foregoing it will be readily evident then that there has been provided an improved getter ion pump whereby the electron paths of the emitted electrons are circurnfused from filament 32 so that they are elongated thereby enhancing the chances of their collision with gas within the space being pumped. Ionization is increased for several reasons in addition to the elongation of the electron paths. For example a larger volume of the space being pumped is swep out and a larger total number of electrons can be made to orbit for a given electron concentration.

I claim:

1. An ionic vacuum pump comprising an outer cylindrical electrode adapted to receive a surface application of getter material, an inner cylindrical electrode concentrically disposed within said outer electrode, said inner electrode being adapted to be maintained at a positive potential with respect to said outer electrode to provide imaginary concentric cylinders of equal potential, the potential of each becoming progressively less with radially outward displacement from said inner electrode, a source of said getter material to be sublimated by electron bombardment disposed around said inner electrode, and a source of electrons serving to ionize gas within the space defined by said outer cylindrical electrode and to bombard said source of getter material, said electron source comprising electron emission means serving to circumfuse electrons into the space defined by said outer electrode, said emission means including means for limiting the circumfusion of electrons primarily to planes substantially normal to the axis of the inner electrode.

2. An ionic vacuum pump comprising an outer cylindrical electrode adapted to receive a surface application of getter material, an inner cylindrical electrode concentrically disposed within said outer electrode, said inner electrode being adapted to be maintained at a positive potential with respect to said outer electrode to provide imaginary concentric cylinders of equal potential, the potential of each becoming progressively less with radially outward displacement from said inner electrode, a source of said getter material to be sublimated by electron bombardment disposed around said inner electrode, and a source of electrons serving to ionize gas Within the space defined by said outer cylindrical electrode and to bombard said source of getter material, said electron source comprising an elongated filament adapted to be maintained at a predetermined filament potential, the axis of said filament being spaced substantially parallel to the axis of said inner cylinder, electrode means coaxially encircling said filament and adapted to be maintained at a positive potential with respect to the filament to cause omnidirectional electron emission from the filament, said electrode means serving to initially restrict said emission to planes substantially normal to the axis of the inner electrode, the axis of said electrode means being disposed substantially on the imaginary surface of that one of said imaginary cylinders having a potential substantially corresponding to the potential of said electrode means to thereby minimize disturbance of the potential gradient of the field radially between said inner and outer electrodes.

3. An ionic vacuum pump according to claim 2 wherein the axis of said electrode means is radially displaced from the axis of said inner cylinder a distance on the order of ths of the radial displacement of the inner surface of the outer cylinder fro-m the axis of the inner cylinder.

4. An ionic vacuum pump according to claim 2 wherein said electrode means is substantially continuously mutually disposed at roughly right angles to said filament.

5. An ionic vacuum pump according to claim 4 wherein said filament comprises a strand of wire formed as a helix of predetermined hand and said electrode means comprises another strand of wire formed as a helix of opposite hand to that of said filament, the electrode means and filament being coaxially disposed, the pitch of the two helices serving to dispose the strands of wire substantially continuously mutually at substantially right angles to one another throughout their coaxial extent.

6. An ionic vacuum pump according to claim 4 wherein said electrode means comprises a plurality of annular conductive rings spaced longitudinally along the axis of the filament, conductive support elements fixed to the rings to maintain the rings in parallel planes, and wherein said filament comprises an elongated linear strand of wire folded upon itself and disposed coaxially within said electrode means.

7. An ionic vacuum pump according to claim 2 wherein the radius of the last named imaginary cylinder is substantially y ths of the radius of the inner surface of said outer cylindrical electrode.

8. An ionic vacuum pump comprising an outer cylindrical electrode adapted to receive a surface application of getter material, an inner cylindrical electrode concentrically disposed within said outer electrode, said inner electrode being adapted to be maintained at a positive potential with respect to said outer electrode to provide imaginary concentric cylinders of equal potential, the potential of each becoming progressively less with radially outward displacement from said inner electrode, a source of said getter material to be sublimated by electron bombardment disposed around said inner electrode, and a source of electrons serving to ionize gas within the space defined by said outer cylindrical electrode and to bombard said source of getter material, said electron source comprising a filament and an encircling control electrode serving to provide circumfusion of electrons from the filament into the space between said inner and outer electrodes and to limit the circumfusion initially primarily to planes substantially normal to the axis of said inner electrode.

9. An ionic vacuum pump comprising an outer electrode formed as a surface of revolution about a predetcrmined axis, said electrode being adapted to receive a surface application of getter material, an inner electrode concentrically disposed within said outer electrode, said inner electrode being adapted to be maintained at a positive potential with respect to said outer electrode to provide concentric imaginary surfaces of revolution of equal potential, the potential of each said imaginary surface of revolution becoming progressively less with radially outward displacement from said inner electrode, a source of said getter material to be sublimated by electron bombardment disposed around said inner electrode, and a source of electrons serving to ionize gas within the space defined by said outer electrode and to bombard said source of getter material, said electron source comprising electron emission means serving to circumfuse electrons into the space defined by said outer electrode, said emission means including a source of electrons and a field control electrode adapted to be maintained at a positive potential with respect to the source of electrons, said control electrode being disposed substantially on that one of the imaginary surfaces of revolution having a potential substantially corresponding to the potential of said control electrode to thereby minimize disturbance of the potential gradient of the field extending between said inner and outer electrodes.

10. Apparatus according to claim 9 wherein said surface of revolution includes right cylindrical portions.

11. Pumping apparatus comprising first and second substantially concentric electrodes defining a space therebetween, the first electrode being encircled by getter material, means for biasing the first electrode positive with respect to the second electrode to form concentric imaginary surfaces of equal potential therebetween, the potential of each said imaginary surface becoming progressively less with radially outward displacement thereof from the inner electrode, means for circumfusing electrons within the space between said first and second electrodes to bombard said first electrode and sublimate getter material therefrom onto said second electrode, and means for restricting said circumfusion of electrons primarily to planes substantially normal to the axis of said electrodes.

12. Pumping apparatus comprising first and second substantially concentric electrodes defining a space therebetween, the first electrode being encircled by getter material, means for biasing the first electrode positive with respect to the second electrode to form concentric imaginary surfaces of equal potential therebetween, the potential of each said imaginary surface becoming progressively less with radially outward displacement thereof from the inner electrode, means for circumfusing electrons through a control electrode element into the space between said first and second electrodes from a predetermined position therebetween with suflicient momentum to bombard said first electrode and sublimate getter material therefrom onto said second electrode, and means for maintaining said electrode element at a predetermined potential substantially corresponding to the potential of that one of said imaginary surfaces which includes said predetermined position.

References Cited UNITED STATES PATENTS 2,131,897 1O /l938 Malter 23069 2,988,657 6/1961 Klopfer et al. 3l37 3,176,907 4/1965 Redhead 23069 3,244,969 4/1966 Herb et al. 32433 ROBERT M. WALKER, Primary Examiner. 

1. AN IONIC VACUUM PUMP COMPRISING AN OUTER CYLINDRICAL ELECTRODE ADAPTED TO RECEIVE A SURFACE APPLICATION OF GETTER MATERIAL, AN INNER CYLINDRICAL ELECTRODE CONCENTRICALLY DISPOSED WITHIN SAID OUTER ELECTRODE, SAID INNER ELECTRODE BEING ADAPTED TO BE MAINTAINED AT A POSITIVE POTENTIAL WITH RESPECT TO SAID OUTER ELECTRODE TO PROVIDE IMAGINARY CONCENTRIC CYLINDERS OF EQUAL POTENTIAL, THE POTENTIAL OF EACH BECOMING PROGRESSIVELY LESS WITH RADIALLY OUTWARDLY DISPLACEMENT FROM SAID INNER ELECTRODE, A SOURCE OF SAID GETTER MATERIAL TO BE SUBLIMATED BY ELECTRON BOMBARDMENT DISPOSED AROUND SAID INNER ELECTRODE, AND A 